4.5.5 Directory Search and Implicit Rules

The search through the directories specified in VPATH or with
vpath also happens during consideration of implicit rules
(see section Using Implicit Rules).

For example, when a file ‘foo.o’ has no explicit rule, make
considers implicit rules, such as the built-in rule to compile
‘foo.c’ if that file exists. If such a file is lacking in the
current directory, the appropriate directories are searched for it. If
‘foo.c’ exists (or is mentioned in the makefile) in any of the
directories, the implicit rule for C compilation is applied.

The recipes of implicit rules normally use automatic variables as a
matter of necessity; consequently they will use the file names found by
directory search with no extra effort.